[intelpt] Refactor timestamps out of `IntelPTInstruction`

Storing timestamps (TSCs) in a more efficient map at the decoded thread level to speed up TSC lookup, as well as reduce the amount of memory used by each decoded instruction. Also introduced TSC range which keeps the current timestamp valid for all subsequent instructions until the next timestamp is emitted.

Differential Revision: https://reviews.llvm.org/D122603